The invention is directed to a method for manufacturing a cost-reduced, durable three-way catalyst useful to oxidize hydrocarbons, carbon monoxide and reduce nitrogen oxides in exhaust gas generated by a gasoline internal combustion engine operated near the stoichiometric A/F ratio. More particularly, the catalyst is made by a process which comprises mixing particles of three oxide materials, ceria-zirconia particles impregnated with platinum and palladium, ceria-zirconia particles impregnated with only rhodium as the precious metal, and a precious metal free gamma-alumina particle.
Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into nonpolluting gases including carbon dioxide, moisture (H2O), and nitrogen. When the gasoline powered engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals like platinum, palladium and rhodium are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often called xe2x80x9cthree-wayxe2x80x9d catalysts. Typically such catalysts use a relatively high loading of precious metal to achieve the high conversion efficiency required to meet stringent emission standards of many countries. This makes the catalyst expensive. In countries where the emission standards are less stringent, a durable catalyst which would meet these less stringent standards and also be less expensive catalyst would be desirable.
We have now found a method for making a durable three-way catalyst which may use a significantly lower loading of precious metal than conventional catalysts making it less expensive, but which still obtains excellent exhaust gas conversion efficiency under close to stoichiometric conditions. This and other aspects of the invention will be discussed in detail below.
The invention is a method for manufacturing a durable, lower cost three-way catalyst useful for treating gasoline engine exhaust gases containing hydrocarbons, carbon monoxide, and nitrogen oxides (NOx). The catalyst has a relatively high loading of rare earth metals as compared to a low loading of precious metals which reduces its cost. The manufacturing method comprises mixing particles of three different materials together. One particle material is: (a) calcined ceria/zirconia particles, having a 20:1 to 1:1 Ce:Zr atomic ratio, impregnated with 1-20 wt. % two precious metal consisting essentially of platinum (Pt) with palladium (Pd) based on the weight of the impregnated particle, preferably this total precious metal loading is 3-8 wt. The second material is (b) calcined ceria/zirconia particle, having a 20:1 to 1:1 Ce:Zr atomic ratio, impregnated with 1-20 wt. % precious metal consisting essentially of only rhodium (Rh) based on the weight of the impregnated particle, preferably this precious metal loading is 3-8 wt. The third particle is: (c) gamma-alumina particles having a particle size, on average, of less than 5 xcexcm. This alumina particle is not impregnated with precious metal.
These powder particles are combined in amounts so as to provide precious metal of Pt:Pd:Rh of about 3-10:3-10:1 by weight in the catalyst composition. The two particle materials impregnated with precious metal comprise 10-30 (wt) % of the catalyst material mixture, i.e., the mixture of the three kinds of particles. When this catalyst material is washcoated onto a substrate, such as a honeycomb substrate often termed a xe2x80x9ccatalyst brickxe2x80x9d, the catalyst material mixture preferably comprise about 10-20(wt) % of the total weight of the substrate plus the catalyst materials. Preferably, the total precious metal carried on the substrate is 9-17 g/ft3 based on the volume of the substrate, e.g., the brick.
According to another aspect of the invention, it is the catalyst made by the process disclosed above and yet another aspect is the method of treating exhaust gases generated by a stoichiometric gasoline engine with the catalyst by contacting the gas with the catalyst.
The present invention method for manufacturing a durable, low cost three-way catalyst which may be wash coated onto a substrate, such as the honeycomb substrates commonly used in catalytic converters in the automotive industry. The catalyst is a mixture of at least three particles, two of them being ceria-zirconia supports loaded with precious metal and a third not having been loaded with precious metal. This mixture of particles may be made into a slurry and then coated on the substrate. These particles and other aspects are discussed in detail below.
The ceria/zirconia particles may be made by any technique. One preferred technique involves impregnating ceria particles with a solution containing a soluble zirconium salt, water being the preferred liquid. Conveniently, water soluble zirconium compounds like nitrates and chlorides, or their mixtures, may be used. After the impregnation, the particles are dried and calcined in air for forming the ceria/zirconia particles. After drying, e.g., at 120xc2x0 C. for several hours, they would be calcined in air at an elevated temperature, e.g., around 600xc2x0 C. for several hours. The precise temperature and length of time of calcining are not critical. Some specific temperatures and times for certain embodiment of the invention are demonstrated in the examples below. It is believed that during calcining in the presence of air, the zirconium that is deposited on the ceria particle is converted to zirconium oxide in combination with oxygen from the air. Hence, this material is given herein as ceria/zirconia. It may be, however, that some is maintained as zirconium in the ceria particles. Either is considered acceptable and part of the present invention and is considered to be included within the terminology xe2x80x9cceria/zirconiaxe2x80x9d particles as used herein.
These particles are not expected to be a mere physical mixture ceria and zirconia, but rather are considered to bichemically bonded in the lattice through the oxygen atoms and hence they could be considered to be part of the same oxide. Another way to form these particles is by co-precipitation of the metal oxides, according to techniques well known to those skilled in the art in view of the present disclosure. It is believed that the close atomic proximity of the metal atoms within the oxide lattice of these particles, and optimally a relatively uniform dispersion of the oxides, contributes to the improved HC, CO, and NOx efficiency. Neither the truth nor the understanding of this theory is necessary for practice of the invention. It is provided in an attempt to explain the unexpected superior properties of the present invention catalyst.
As seen from the disclosure above, these ceria/zirconia precious metal support particles in their preferred embodiments contain an excess of cerium relative the zirconium. That is, while the Ce/Zr atomic ratio of these particle is in its broadest embodiment 20:1 to 1:1, preferably the ratio is 10:1 to 2:1. As disclosed above, ceria/zirconia particles xe2x80x9caxe2x80x9d are used to support precious metal consisting essentially of platinum together with palladium, while ceria/zirconia particles xe2x80x9cbxe2x80x9d are used to support precious metal consisting essentially only of rhodium. Thus, the rhodium is carried as the sole precious metal on a different particle than carries the platinum/palladium mixture. The Ce/Zr atomic ratio may be the same or different for the particles xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d as is also the case for the amount of precious metal loading on these different particles.
To provide the precious metal on the ceria/zirconia particles, any technique including the well-known wet impregnation technique from soluble precious metal precursor compounds may be used. Water soluble compounds are preferred, including, but not limited to nitrate salts and materials for platinum like chloroplatinic acid. As is known in the art, after impregnating the washcoat with the precursor solution, it is dried and heated to decompose the precursor to its precious metal or precious metal oxide. As is known in the art, the precursor may initially decompose to the metal but be oxidized to its oxide in the presence of oxygen. While some examples of precious metal precursors have been mentioned above, they are not meant to be limiting. Still other precursor compounds would be apparent to those skilled in the art in view of the present disclosure. In addition to this incorporation from a liquid phase, the precious metal, such as the platinum, may be provided by sublimation of platinum chloride or other volatile platinum salts: by solid state exchange in the 300-500xc2x0 C. temperature range using labile platinum compounds. There is no criticality to the type of precursor compound that may be used to provide the precious metal according to this invention. The platinum and/or palladium that are provided may be provided separately or as a mixture, as from a single solution mixture of the two, but in the latter case rhodium would not be included in the mixture. Rather, as discussed above, the rhodium is provided separately on the other ceria/zirconia particles as mentioned above. That is, the rhodium is impregnated on different particles than those carrying the platinum/palladium. This is critical to the present invention. Palladium has a relatively low sulfur tolerance, while rhodium has better reducing reactivity for NOx reduction. Thus the separation of rhodium from platinum/palladium helps to maintain the catalyst activity especially towards the NOx reduction even with relatively high sulfur content in the gasoline fuel. Neither the truth nor understanding of this theory is necessary however for practice of the present invention.
As disclosed above, the present invention method provides a durable catalyst which is able to efficiently convert exhaust gas components using a catalyst which may contain significantly less precious metal and at hence lower cost than conventional highly loaded three-way precious metal catalysts. This is believed to be the result of forming the catalyst from pre-made precious metal containing particles, rather than in the conventional way where a single powder is simply impregnated with all of the precious metals. In this latter conventional case, the precious metal would be dispersed evenly inside the washcoat and the reactive sites could be buried especially under the thermal attack experienced in actual exhaust system operation. In the present invention, the precious metals are provided onto pre-calcined particles which is believed to allow the reactive sites to remain readily available on the surface to be exposed to the exhaust gases. In addition, under thermal attack, more pores are generated due to the pre-calcined particle structure. Therefore, the exhaust can access the reactive sites more easily, resulting in a high conversion efficiency with the present invention catalyst even with a low precious metal loading. As explained above, neither the truth nor understanding is necessary to the practice of the present invention, however. It is provided in an attempt to explain the excellent properties of the present invention.
Besides the ceria/zirconia particles loaded with precious metal as described above, the invention catalyst particle mixture also includes particles of alumina on which no precious metal has been loaded. The alumina particles may be made from alumina like gamma-alumina as the sole alumina or mixed with other alumina forms, and may include stabilizers in small amounts for the alumina. It is well known in the art to stabilize alumina for high temperature use. Selection of such oxide stabilizers would be well known to those skilled in the art in view of the present disclosure. Examples of such oxide stabilizers include thermal stabilizers like titanium, zirconium or barium oxide, while structural stabilizers include for example, titanium and calcium oxide. The alumina is mixed with the stabilizers, e.g., as by milling in a slurry. The alumina particle size incorporated in the mixture of catalyst particles is optimally less than about 5 microns. In contrast, the ceria/zirconia particles incorporated in the mixture have a relatively larger particle size, that is 5 microns to about 100 microns, most optimally being 10-60 microns. When the particles are mixed and if the mixture is ball milled, the particle size may be reduced.
As disclosed above, in the catalyst particle mixture, three precious metals are present, platinum (Pt), palladium (Pd) and rhodium (Rh). The total platinum/palladium precious metal loading on particles xe2x80x9caxe2x80x9d is broadly 1-20, preferably 3-8 wt. % based on the weight of these impregnated support particles, i.e., the weight of the particles with the impregnated precious metal. The rhodium loading on particles xe2x80x9cbxe2x80x9d is also broadly 1-20 wt. % based on the weight of these impregnated particles, with 3-8 wt. % being preferred. Another way to express this is: (Pt+Pd)/((Pt+Pd)+Ce/Zr)=1-20%(wt), preferably 3-8%(wt) and Rh/(Rh+Ce/Zr)=1-20%(wt), preferably 3-8%(wt).
These precious metal containing particle materials (xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d) are mixed with alumina (particle xe2x80x9ccxe2x80x9d ) to form the catalyst composition. In the catalyst composition, the Pt:Pd:Rh weight ratio is 3-10:3-10:1. When this particle catalyst composition is washcoated on a substrate as would generally be the case for useful application, the precious metal-containing particles comprise 10-30 (wt) % of the washcoat materials, i.e,, ((Pt+Pd+Ce/Zr)+(Rh+Ce/Zr))/((Pt+Pd+Ce/Zr)+(Rh+Ce/Zr)+"Ugr"xe2x88x92Al2O3)=10-30%(wt). And when applied to a substrate, the washcoat material preferably comprises about 10-20 (wt) % of the entire catalyst substrate (brick). ((Pt+Pd+Ce/Zr)+(Rh+Ce/Zr)+gammaxe2x88x92Al203)/((Pt+Pd+Ce/Zr)+(Rh+Ce/Zr)+gammaxe2x88x92Al203+substrate)=10-20%(wt). The optimal total precious metal loading is about 9-17 g/ft3 based on the volume of the substrate. That is, (Pt+Pd+Rh)/(volume of washcoated substrate)=9-17 g/ft3.
In forming the catalyst, the particles are mixed together, optimally in a water slurry, and then applied to the substrate. For example, the alumina particle may be provided in a water slurry and then stabilizers like barium nitrate and magnesium mixed together in a nitric acid solution. This slurry can then be ball milled to form alumina particles of the desired size, generally less than 5 microns. The precious metal impregnated ceria/zirconia particles are then added in and ball milled to provide a uniform mixture of the particles in the slurry. The ball milling of the slurry can be continued to reduce the particle size of the particles, if desired. As the particle size of the washcoat particles decreases, the catalyst is more efficient in contacting the exhaust gas. The slurry is then diluted with water and adjusted to a viscosity suitable for coating on a substrate, at about 3000 cp.
As is known in the art, for useful application of the catalyst in an exhaust gas system, the catalyst is deposited or washcoated on a substrate (mechanical carrier) made of a high temperature stable, electrically insulating material such as cordierite, mullite, etc. A mechanical carrier is preferably comprised of a monolithic magnesium aluminum silicate structure (i.e., cordierite), although the configuration is not critical to the catalyst of this invention. It is preferred that the surface area of the monolithic structure provide 50-1000 meter square per liter structure, as measured by N2 adsorption. Cell density should be maximized consistent with pressure drop limitations and is preferably in the range of 200-800 cells per square inch of cross-sectional area of the structure. The substrate may be in any suitable configuration, often being employed as a monolithic honeycomb structure, spun fibers, corrugated foils or layered materials. Still other materials and configurations useful in this invention and suitable in an exhaust gas system will be apparent to those skilled in the art in view of the present disclosure.
Techniques for providing an oxide washcoat on a substrate is well known to those skilled in the art. Generally a slurry of the mixed metal oxide particles and optionally stabilizer particles is coated on a substrate, e.g., as by dipping or spraying, after which the excess is generally blown off. Then it is heated to dry and calcine the coating, generally a temperature of about 700xc2x0 C. for about 2-3 hours may be used. Calcining serves to develop the integrity of the ceramic structure of the washcoated oxide coating. The total amount of the precious metal/oxide washcoat carried on the substrate is about 10-20 wt. %, based on the weight of the substrate coated. Several coatings of the substrate in the washcoat may be necessary to develop the desired coating thickness/weight on the substrate. The substrate would optimally carry about 9-17 g/ft3 of precious metal based on the volume of the substrate, more preferably being 10-15 g/ft3.
The catalyst is expected to be used in automotive vehicles for emission treatment in the exhaust gas system where it functions to oxidize hydrocarbons, carbon monoxide, and simultaneously reduce nitrogen oxides to desired emission levels as for example in South American countries or in Asian countries. In these applications it is more than sufficient to meet the emission standards. In other situations where the catalyst would not be sufficient by itself to meet emission standards, for example as might be the case in the United States, the invention catalyst may be used in combination with another emission control catalyst. One such application involves the use of the invention catalyst downstream of another three-way catalyst either as a separate stage or as a separate brick in the same converter. Hence, the use of the present invention catalyst is not limited to a particular application.